Synthesis, structural characterization, electrochemical properties and polymerization behaviour of the first silicon-bridged [1.1]ferrocenophane [{Fe(η-C5H4)2SiMe2}2]

Abstract

The first silicon-bridged [1.1]ferrocenophane, [{Fe(η-C₅H₄)₂SiMe₂}₂]5, was prepared in five steps starting from [Fe(η-C₅H₄Li)₂]·tmen (tmen =N,N,N′,N′-tetramethylethylenediamine). Reaction of [Fe(η-C₅H₄Li)₂]·tmen with SiMe₂Cl(NEt₂) afforded [Fe{η-C₅H₄SiMe₂(NEt₂)}₂]6 in high yield. Substitution of the silylamine groups of 6 for chlorine by reaction with acetyl chloride gave the dichloro species [Fe(η-C₅H₄SiMe₂Cl)₂]7 also in high yield. Reaction of 7 with Li(C₅H₅) in tetrahydrofuran produced the intermediate [Fe{η-C₅H₄SiMe₂(C₅H₅)}₂], which was subsequently lithiated and treated with FeCl₂ to yield 5, in moderate yield. A single crystal X-ray diffraction analysis of compound 5 revealed that the two ferrocene halves are in an anti conformation, with a Fe ⋯ Fe distance of 5.171 (9)Å. Cyclic voltammetric analysis of 5 in CH₂Cl₂ showed the presence of two reversible oxidation waves with a separation of 0.25 V consistent with an extensive interaction between the Fe centres. The room-temperature ⁵⁷Fe Mössbauer spectrum of 5 is similar to that of 1,1′-bis(trimethylsilyl)ferrocene with an isomer shift of 0.44(1) mm s^–1 and a quadrupolar splitting value of 2.32(1) mm s^–1. Compound 5 was found to be resistant to thermal ring-opening polymerization and attempted copolymerization with the strained silicon-bridged [1]ferrocenophane [Fe(η-C₅H₄)₂SiMe₂] also proved unsuccessful.